The present invention relates to a process for making a workpiece suitable for use in a semiconductor device, and more particularly a workpiece comprising a thermally and electrically conductive lead and a refractory metal contact joined by a high temperature brazing process.
Passivated semiconductor devices generally include a semiconductor body composed substantially of silicon, a layer of passivating material such as glass or plastic disposed about the semiconductor body, and at least one metallic contact extending outwardly from the semiconductor body through the passivating layer as an external contact for connection with the associated circuitry. More specifically, such devices require that the metallic contacts be refractory in nature in order that the coefficients of thermal expansion of the semiconductor body, the passivating layer and the metallic contacts be reasonably matched to avoid breakage during thermal cycling. Molybdenum, tungsten, tantalum and various special alloys are typical of the refractory metals used as such refractory metal contacts; however, since such materials are both expensive and relatively poor conductors of both heat and electric current, the refractory metal contacts are generally joined to good conventional conductors (such as copper, silver or various special alloys) just beyond the passivating layer, the connection between the refractory contact and the other circuitry elements being made by the conventional conductor. In the case of axial-lead-construction semiconductor rectifiers, the connection of the conventional axial lead to the refractory metal contact is accomplished by one of the following two procedures.
In the first procedure, the refractory material is initially plated with a solderable metal such as silver before application of the passivating layer. After the passivating operation, the axial leads are attached to the refractory metal contacts using "soft solder" preforms with melting points typically less than 350.degree. C. Devices containing such connections have the disadvantages commonly associated with soft solder contacts. If extreme temperature control is not exercised in soldering the axial lead to other circuit components, the axial lead may detach from the refractory metal contact as the soft solder heats up. In any case, it has been found that such soft solder joints are subject to thermal fatigue and a resultant short operating life.
In the other procedure, the refractory metal contact is joined to the axial lead by a special welding process known as butt or percussion welding. The joint and the axial lead must thereafter be exposed to all the chemical and heat-treating processes subsequently required to (1) join the semiconductor body to the refractory metal contact, (2) etch the subassembly, and (3) apply and fuse the passivating glass layer. The devices fashioned according to this procedure are not reliable in the first place because a true weld is not possible between the refractory metal of the contact and the conventional conductive metal of the axial lead. Furthermore, the processing operations required to complete the semiconductor device subsequent to formation of the refractory contact/axial lead joint frequently result in a weak and porous joint which will develop high electrical and thermal resistance in time or eventually even fail mechanically and fall off. Experience has shown that butt welded joints have extremely high failure rates when exposed to conditions of high temperature and high humidity, the failure rate rising as high as 50% at conditions of 85.degree. C and 85% relative humidity.
Yet another disadvantage of the process of joining a refractory metal contact to an axial lead by the butt or percussion welding technique is that certain details of the technique are proprietary information, not readily available on the open market. But even if the full details of this technique were generally known, its very nature imposes undesirable constraints on the design of the refractory metal contacts and the axial leads useful therein. In the technique, the axial lead is actually in motion as the weld between the axial lead and the refractory metal contact is made, the parts being brought together to make the electrical contact by rapid advance of the axial lead once the refractory metal contact is in position. The technique requires that each refractory metal contact be inserted into an individual clamp-designed welding electrode in order to control the welding current. Thus, for the automatic rapid positioning essential to mass production, it is necessary that the dimensions of the refractory metal contact adhere to certain rigid specifications, and in particular that it be of a length sufficient to be grasped by the welding electrode clamp. To insure reasonable centering of the axial lead relative to the end of the refractory metal contact, the end of the axial lead must generally be at least 0.5 mm. smaller in diameter than the end of the refractory metal contact. Thus the technique requires a minimum cross-sectional area for the refractory metal contact and additionally imposes a maximum cross-sectional area for the axial lead for a given refractory metal contact. These length and cross-sectional limitations restrict the degree of miniaturization which can be obtained in any semiconductor device ultimately embodying the axial lead/refractory metal contact subassembly.
Accordingly, it is an object of the present invention to provide a process for attaching a thermally and electrically conductive lead to a refractory metal contact using a high temperature brazing process.
It is another object to provide such a process which avoids the aforementioned disadvantages of "soft solder" and "butt welded" joints.
It is also an object to provide such a process wherein the resultant joint is of higher physical strength and lower porosity than a butt-welded joint, withstands a higher temperature than a soft solder joint, and withstands conditions of high temperature and high humidity better than a butt-welded joint.
It is a further object to provide such a process which does not impose limitations on the minimum length or cross-section of the refractory metal contact or on the maximum cross-section of the axial lead for a given refractory metal contact and thus lends itself to unimpeded miniaturization of the semiconductor device embodying the axial lead/refractory metal contact subassembly.
Yet another object is to provide such a process which is adapted to mass production techniques.
Still another object is to provide such a process which utilizes a special preform, is simple and economical to perform, is less expensive than butt welding, and provides a joint superior to those produced by soft solder or butt welding processes.
A final object is to provide a brazed lead electrode workpiece adapted for use in a semiconductor device and comprising a lead member joined to a refractory metal contact by the aforesaid brazing process.